1. Field of the Invention
The present invention relates to a snubber circuit and particularly relates to a snubber circuit which suppresses a surge voltage generated in a rectifier circuit of a switching power supply device.
2. Description of the Related Art
In general, a switching power supply device includes a transformer which includes a primary winding and a secondary winding; a first switching element which intermittently applies a DC power supply voltage to the primary winding; a rectifier circuit which is connected to the secondary winding; and a smoothing circuit which includes a first coil connected subsequently to the rectifier circuit. In such a switching power supply device, since a surge voltage is generated in the rectifier circuit when the voltage of the secondary winding rises, a snubber circuit which suppresses a surge voltage is connected to the rectifier circuit.
As the snubber circuit, there is a circuit which includes a second switching element and a capacitor which are connected in series between output nodes of the rectifier circuit; a second coil which is electromagnetically coupled to the first coil; and a differentiating circuit which differentiates an output voltage of the second coil and applies the differentiated voltage to a control terminal of the second switching element. In the snubber circuit, the second switching element is switched ON at the timing when a surge voltage is generated in the rectifier circuit, and the energy of the surge voltage is stored in the capacitor and then released (see, for example, Japanese Unexamined Patent Application Publication No. 1-202161).
However, in an existing snubber circuit, depending on the configurations of first and second coils, as shown in FIG. 5, when a voltage between terminals of the second coil is raised to a positive voltage (time t0), a vibration voltage is generated between the terminals of the second coil, and a control voltage Vgs having the same waveform as that of the vibration voltage is applied to the control terminal of a second switching element. The control voltage Vgs attenuates over time while vibrating.
When a threshold voltage for the second switching element is denoted by VTH, the second switching element is switched ON during a period of Vgs>VTH, and the second switching element is switched OFF during a period of Vgs<VTH. Therefore, the second switching element repeatedly switches ON/OFF, it is impossible to switch the second switching element ON at a predetermined timing, and it is impossible to sufficiently suppress a surge voltage.